Zinc ion-triggered two-way macro-/microscopic shape changing and memory effects in high strength hydrogels with pre-programmed unilateral patterned surfaces

We demonstrate that dipole-dipole reinforced (DDR) high-strength hydrogels with pre-programmed unilateral patterned surfaces exhibit a two-way reversible shape-changing and memory effect caused by dynamic association and dissociation of CN-CN dipole-dipole pairings uniquely responding to zinc ions. The curling geometry of the gel can be controlled by the direction of the stripes on the surface of the DDR hydrogel. Interestingly, annealing is shown to facilitate folding of hydrogel; especially annealing at selective regions of one single gel leads to a more complex shape changing effect. More strikingly, surface wrinkling can be conveniently generated by heat treatment of bilayer DDR hydrogels composed of a hierarchical structure, with the micropatterns exhibiting reversible "shape-memorizing'' behavior driven by zinc ions. This strategy offers a facile approach to fabricating a promising mechanically strong soft-wet actuator and rewritable hydrogel surface.